Drilling Fluids

Circulatory System

Mud Engineer

• Mix the drilling fluid• Maintain the properties of

the drilling fluid• Inventory management of

mud chemicals and supply

Functions of Drilling Fluid

1. Remove drilled cuttings from the borehole2. Carry and release the cuttings at the

surface3. Suspend cuttings and weight material in

suspension when circulation is stopped4. Release cuttings when processed by

surface equipment5. Allow cuttings to settle out at the surface

Functions of Drilling Fluid

6. Control subsurface pressures7. Prevent the borehole from collapsing or

caving in8. Protect producing formations from

damage that could impair production9. Clean, cool, and lubricate the drill bit and

drillstring. This is the simplest and one of the most essential duties of drilling mud

Functions of Drilling Fluid

10.Seal porous and permeable zones with an impermeable filter cake

11.Help support part of the weight of the drillstring/casing

12.Ensure maximum information from the formation drilled

13.Do everything, without damage to the circulation system or upon the formation adjacent to the hole

Drilling Fluid Properties

• Mud weight

• Viscosity

• Gel strength

• Water loss

Mud Weight

• MW must be sufficient to contain subsurface pressures and to prevent the formation from caving in

• Not practical or economical to have the mud weight too high– Low ROP– Fracturing of weak formation -> lost circulation

Mud Weight: Effect on Mud Logging

• Excessive MW -> might cause a loss of circulation -> possible contamination or loss of cuttings

• MW < Pore Pressure -> kick or sloughing of hole or even worse stuck pipe -> loss of samples or abundant cavings

Lost Circulation

• The lack of drilling fluid returning to the surface after being pumped down a well

• Due to: – Fissures, caverns, fractures– Enlarged hole size– Excessive mud weight

• Terms used to describe degree of loss:– Seepage– Partial lost returns / circulation– Total lost returns / circulation

Viscosity

• It is the resistance to flow• It affects the ability of the drilling fluid to lift the

cuttings out of the borehole• It is dependent on the amount and character of the

suspended solids• Measured in the field using a Marsh Funnel• The measurement of Funnel Viscosity is sec/qt

(seconds per quart)

Gel Strength• Refers to the ability of the drilling fluid to develop a gel as

soon as it stops moving• Determined with a Fann VG (Viscosity/Gel) Meter

• Expressed in lbs/100ft2 • Its purpose is to suspend the cuttings and mud solids (weight

material) in the borehole and not permit them to settle around the bit when circulation is halted

• Gel strength should be low enough to– Allow the cuttings to be removed at the surface– Permit entrained gas to be removed at the surface– Minimize swabbing when the pipe is pulled from the borehole– Permit starting of circulation without high pump pressures

Effects of Gel Strength On Mud Logging

• If the viscosity or gel strength (or both) is too high, the entrained gas may be recycled several times. Swabbing of the borehole may also introduce extraneous gas anomalies.

• Fine cuttings may be held in suspension so they cannot be removed at the shale shakers and settling pits, thus recycling and contaminating the cuttings samples. Also, cuttings consisting of clays or other dispersible material may be dissolved.

Water Loss

• Control of the filtration rate (water loss) is necessary for two reasons:– A poor quality filter cake may cause excessive

water loss and produce an excessively thick filter cake

– High water loss can cause deep invasion of the formations, making it difficult to interpret wireline logs

Pressure Control Terminology

• Hydrostatic Pressure

• Pressure Gradient

• Equivalent Mud Weight

• Pore Pressure

Hydrostatic Pressure

• The pressure that exists due to the mud weight and vertical depth of the column of fluid

• The size and shape of the fluid column have no effect

• Formula used:– Hp or P (psi) = 0.519 x MW (lbs/gal) x TVD (feet)

– Hp or P (bars) = 0.0981 x MW (g/cc) x TVD (meters)

Pressure Gradient

• It is the rate of change of hydrostatic pressure with depth for any given unit of fluid weight

• Formula:– Pressure Gradient = 0.0519 x MW

• Typical formation water density used is 8.6 ppg yielding a pressure gradient of 0.446 psi/ft

Equivalent Mud Weight

• The mud weight that would exert a hydrostatic pressure equal to the sum of the imposed pressure and the hydrostatic pressure

• Used interchangeably with Equivalent Circulating Density (ECD)

• Calculated by DLS computer using Power Law • MWe = MWo + Pi/(0.0519 x TVD)

– where, MWe = Equivalent MW; MWo = Original MW; Pi = imposed pressure

Pore Pressure

• Pore pressure is the pressure that is exerted by fluids contained in the pore space of the rock and is the strict meaning of what is generally referred to as formation pressure

Types of Drilling Fluids (EDIT2)

• Water-based muds

• Oil-based muds

• Synthetic muds

• Air, Gas, Mist systems

Types of Water-based Muds (NEW2)• Non-dispersed

• Dispersed

• Calcium-treated

• Polymer muds

• Low-solids

• Saltwater

Non-Dispersed (NEW2)

• Include spud muds, natural muds and other lightly treated systems generally used for shallow wells or top-hole drilling.

• Thinners and dispersants are not added to disperse drill solids and clay particles.

Dispersed (NEW2)

• Muds are often dispersed by deflocculants / filtrate reducers (typically lignosulfonates, lignites or tannins). Potassium-containing chemicals are frequently used to provide greater shale inhibition.

Calcium-treated (NEW2)

• High levels of soluble calcium are used to control sloughing shale and hole enlargement, and to prevent formation damage.

• Hydrated lime (calcium hydroxide), gypsum (calcium sulfate) and calcium chloride are principal ingredients of calcium systems.

• Calcium treated muds resist salt and anhydrite contamination but are susceptible to gelation and solidification at high temperatures.

Polymers (NEW2)

• Muds incorporating generally long-chain, high-molecular-weight polymers are utilized to either encapsulate drill solids to prevent dispersion and coat shales for inhibition, or for increasing viscosity and reducing fluid loss

• These systems normally contain a minimum amount of bentonite and may be sensitive to divalent cations such as calcium and magnesium.

• Most polymers have temperature limits below 300[degrees]F, but under certain conditions, may be used in wells with appreciably higher BHTs.

Low Solids (NEW2)

• Total solids should not range higher than about 6% to 10% by volume. Clay solids should be some 3% or less and exhibit a ratio of drilled solids to bentonite of less than 2:1.

• Low solids systems typically use polymer additive as a viscosifier or bentonite extender and are non-dispersed.

• One primary advantage of low-solids systems is that they significantly improve drilling penetration rate.

Saltwater Systems (NEW2)

• Saltwater systems have a chloride content of 10,000 to 190,000 mg/l. The lower levels are usually referred to as brackish or seawater systems.

• Saltwater muds are usually prepared from brackish, seawater or produced-water sources.

• Saturated salt systems have a chloride concentration near 190,000 mg/l (saturated) and are used to drill salt formations.

Components of a Water-Based Mud

• Continuous liquid phase of water in which clay materials are suspended

• Reactive solids

• Inert solids

Water-Based Mud: Water Component

• Either: fresh or saltwater

• Seawater commonly used in offshore drilling

• Saturated saltwater is used for shale inhibition and drilling thick evaporite sequences

Water-Based Mud: Reactive Solids Component

• Clays (“gel”)

• Dispersants

• Filtration Control Agents

• Detergents, Emulsifiers and Lubricants

• Defoamers

• Sodium and Calcium compounds

Reactive Solids: Clays

• Basic material of mud• Sometimes referred to as “gel”• Affects viscosity, gel strength and water loss• Common clays are:

– Bentonite - for fresh water muds

– Attapulgite - for saltwater muds

– Natural formation clays that hydrate and enter the mud system

Reactive Solids: Dispersants/Deflocculants

• Reduce viscosity by adsorption onto clay particles, reducing flocculation

• Also aid in filtration control since filtration problems are related to flocculation

Flocculation

• Thickening of the mud due to the attraction of clay platelets in the mud

• Causes:– High active solids concentration– High electrolyte concentration– High temperature

• Detected by:– Abnormally high yield point– Abnormally high gel strength

Examples of Dispersants

• Tannins (extracted from the quebracho and hemlock trees)

• Phosphates (up to depths <175 F BHT)

• Lignite

• Lignosulphonates (obtained from spent sulfite liquor generated during paper manufacturing)

Reactive Solids: Filtration Control Agents

• Control the amount of water loss into permeable formations by ensuring the development of a firm impermeable filter cake

• Commonly used filtration control agents:– Starch (used in muds with high salt content)– Sodium Carboxy-Methyl Cellulose (CMC)– Polymers

Reactive Solids: Detergents, Emulsifiers and Lubricants

• Assist in cooling and lubricating

• Also used for a spotting fluid in order to free stuck pipe

Reactive Solids: Defoamers

• Used to prevent mud foaming at the surface in mud tanks or surface cleaning equipment

Reactive Solids: Sodium and Calcium Compounds

• Sodium Compounds precipitate or suppress calcium or magnesium that decreases the yield of the clays.

• Calcium Compounds inhibit formation clays and prevent them from hydrating or swelling.

Water-Based Mud: Inert Solids Component

• Weight Material

• Lost Circulation Material (L.C.M.)

• Anti-friction Material

Inert Solids: Weight Material

• Finely ground, high-density minerals held in suspension to control mud density

• Common weight materials are:– Barite– Hematite– Galena

Inert Solids: Lost Circulation Material

• Used to plug the lost circulation zone• If none of the LCM materials successfully plug the

lost circulation zone, the zone must be cemented off• Common LCM materials:

– Fibrous: wood fiber, leather fiber– Granular: walnut shells (nut plug), fine, medium, coarse– Flakes: cellophane, mica (fine, coarse– Reinforcing Plugs: bentonite with diesel oil (gunk), time

setting clays, attapulgite and granular (squeeze)

Inert Solids: Anti-Friction Material

• To reduce torque and decrease the possibility of differential sticking

• More frequently it is used on high angle directional wells, where torque and differential sticking are a problem

• Most frequently used materials are inert polyurethane spheres

Low Solids Mud System

• Solids content is less than 10% by weight or the mud weight is less than 9.5 ppg

• It may either have a water or oil base

Oil-Based Muds (EDIT 2)

• Used for a variety of applications where fluid stability and inhibition are necessary, such as high-temperature wells, deep holes and where sticking and hole stabilization are problems.

• Their high cost and difficulty of running, and complication of geological evaluation preclude their use on exploratory wells, other than in certain troublesome evaporite and clay sections

4 Types of “Oil-Based” Muds

• Emulsion (oil/water) System

• Invert Emulsion (water/oil) System

• Oil mud

• True Oil-based mud

Emulsion (Oil/Water) System

• Diesel or crude oil is dispersed in a continuous phase of water

Invert Emulsion (Water/Oil) System (EDIT2)• Invert emulsion muds are water-in-oil

emulsions typically with calcium chloride brine as the emulsified phase and oil as the continuous phase.

• May contain up to 50% by volume of water in the liquid phase

• Now synonymous with oil based mud

Oil Muds

• The IADC identifies oil muds as, “usually a mixture of diesel fuel and asphalt; not emulsions at the start of their use in drilling.

Oil Base Muds

• Oil-base mud contains blown asphalt and usually 1 to 5% water emulsified into the system with caustic soda or quick lime and an organic acid. Oil-base muds are differentiated from invert emulsion muds (both water-in-oil emulsions) by the amounts of water used, method of controlling viscosity and thixotropic properties, wall-building properties and fluid loss.”

Synthetic Muds (SBM) (NEW2)

• Synthetic fluids are designed to mirror oil-based mud performance, without the environmental hazards.

• Primary types of synthetic fluids are esters, ethers, poly alpha olefins and isomerized alpha olefins. They are environmentally friendly, can be discharged offshore and are non-sheening and biodegradable.

Air, Mist, Foam and Gas Systems (EDIT 2)• Used principally in hard clay or rock

formations or in areas where there is little formation water

• At times a foaming agent is used to improve carrying capacity

Air Drilling

• A compressor assembly including cooling system, air receiver and unloading system replaces the mud pump

• The air line is connected to the swivel hose at the top of the kelly or top drive.

• A small stream of water is often introduced into the air system to help cool the drill bit and control dust

End of Topic